The present invention relates to electronic semiconductor devices, and, more particularly, to Schottky barriers with gallium arsenide and related semiconductors.
The contact between a metal and a semiconductor creates an electrical barrier at the interface (Schottky barrier), and the barrier height depends on factors such as the work functions of the metal and semiconductor, doping level of the semiconductor, surface state density, and so forth; see, generally, Sze, Physics of Semiconductor Devices, ch.5 (John Wiley and Sons, 2d Ed, 1981). In low power logic circuits based on normally-off (enhancement mode) MESFETs, a positive voltage must be applied to the gate electrode to turn on the device (that is, the built in voltage of the gate-channel interface is large enough to deplete the channel with no applied gate voltage). But the gate voltage is limited roughly to a few kT below the barrier height in order to limit gate current (about 0.6 V for titanium-platinum gates on gallium arsenide); and this limits the possible logic swing and noise margin of the logic circuits. Thus it is necessary to have a large barrier height in order to have a large logic swing which is important for noise immunity.
However, for III-V semiconductors such as gallium arsenide, gallium antimonide, and indium phosphide the metal-semiconductor interfaces have barrier heights essentially independent of the metal's work function due to a pinning of the Fermi level by the semiconductor's surface states. For example, gallium arsenide forms contacts with metals such as platinum, palladium, aluminum, and tungsten that all have a barrier height of about 0.8 V; and this barrier height is too low for good enhancement mode MESFETs. Thus it is a problem to increase the barrier heights of metal contacts with gallium arsenide and similar semiconductors.
Attempts have been made to vary barrier heights in metal-semiconductor contacts, such as by varying doping concentration, but only barrier height lowering results. Also, the use of a thin insulator raises the effective barrier height, but the resulting diodes are non-ideal as the probability of tunneling through the isulator depends strongly on the applied voltage. A known technique of increasing the barrier height of a metal-semiconductor contact is insertion of a thin layer of oppositely doped semiconductor between the metal and the semiconductor. For example, if a layer 100 .ANG. thick of p+ silicon is inserted between nickel and n type silicon (such as by implantation of antimony followed by annealing prior to deposition of the nickel), then the barrier height increases approximately linearly with the antimony dose; see J. Shannon, Control of Schottky Barrier Height Using Highly Doped Surface Layers, 19 Solid State Electronics 537 (1976). However, these techniques have had limited success due to the lack of control of the metal-semiconductor interface, and the lack of control over the doping concentration and the abruptness of the junction.
These limitations may be overcome by the use of molecular beam epitaxy (MBE). For example, Eglash et al, 22 Jpn.J.Appl.Phys.Suppl. 431 (1983), grew layers of p+ type gallium arsenide with thicknesses in the range of 50 to 360 .ANG. and doped with acceptor concentrations in the range of 1.times.10.sup.18 to 1.times.10.sup.20 per cm.sup.3 on n type gallium arsenide (donor concentrations 5.times.10.sup.16 per cm.sup.3) and contacted the p+ layer with aluminum; the barrier height was raised from 0.79 V to 1.24 V.
But MBE is a relatively slow and expensive process to form the thin, oppositely doped layer and not adapted to high throughput, and implantation of p type dopants such as zinc and beryllium in gallium arsenide to form the thin oppositely doped layer fails due to the rapid diffusion of the dopants during the anneal subsequent to the implantation. Thus it is a problem of the known barrier height raising techniques to achieve simple processing steps with high throughput.